Wednesday, July 11, 2018

Twitching Spider Leg

As I was sitting at my desk last week, I saw a little brown spider come out from behind some books.  I suspected the spider was a brown recluse so I used a pencil to pin down its legs while looking for an insect box to put it in.  Two of its legs fell off and started twitching, continuing for over two minutes while I got my camera to take this video.

Few spiders can be identified down to species just by their markings.  Even black widows have several species, separated by the details of their hourglass.  The identification of the brown recluse, aka fiddle spider, is "confirmed" by the fiddle marking on the dorsal thorax above.  At least, that is what we were all taught.
The Burke Museum calls this a myth as there are other spiders that can have fiddle shaped markings.  Another myth involves an outbreak of L. reclusa  bites in California as this species only occurs in the south-central United States.  Spider identification can be narrowed down by the eyes.

Spider identification is tricky at best but it usually begins by looking at the arrangement of their eyes and comparing it to this chart.  For those of us non-arachnologists, looking one in the eye involves either a smashed spider or a camera.  In this case it was this postmortem view that put it in the Loxosceles genera, called the brown spiders.  There are eleven species of Loxosceles in the United States, but only  Loxosceles reclusa is reported in Missouri.

Certain identification down to species requires a detailed examination of an adult spider's genitalia under a microscope.  And even the "fiddle" requires some magnification, something I can't do with a moving spider, so all brown spiders of that certain size are brown recluse until proven otherwise at postmortem. 

Exposed "hip joints"
Back to the twitching leg, this is a common finding when Opiliones (harvestmen) lose a leg and has been reported with Pholcids (cellar spiders).  Both groups have extremely long legs and tiny bodies, meaning that predators are most likely to grab a fragile leg.  Cowles describes this as a "flexible secondary defense." *  The leg kicking around on its own entertains the predator and may actually give it a satisfying treat while the victim makes an escape, similar to the twitching blue tail of a five-lined skink.

Opioliones actually have muscles in the trochanter that contract to prevent bleeding.  "Two pacemaker nerve ganglia within the leg are activated as soon as communication with the central nervous system is severed.  They operate independently, causing the leg to twitch and jerk.  The legs even have their own trachae which supply the disembodied legs with oxygen.  The leg can twitch for up to an hour, as opposed to 40 seconds without oxygen."*

I can't find any information on brown recluse spiders having disconnected twitching legs but I have the video and seeing is usually believing.
* Amazing Arachnids, Jillian Cowles.